This invention relates generally to fluorescent lamps and more particularly to cathodes used in fluorescent lamps.
Fluorescent lamps include a sealed glass tube that contains a small amount of mercury and an inert gas, such as argon, neon or the like, kept under very low pressure. The inside surface of the glass tube is coated with a phosphor powder that fluoresces when excited. A typical fluorescent lamp has a cathode (also referred to as a coil or an electrode) mounted inside the tube at each end thereof, although single-ended lamps are also known. The cathodes are coated with an emitter material that emits electrons during lamp operation. When the lamp is on, alternating current flows through the cathodes producing a voltage across the cathodes. This causes electrons to migrate through the gas from one end of the tube to the other. These electrons collide with mercury atoms, causing the mercury atoms to be ionized and excited. When the mercury atoms return to their normal state, photons corresponding to mercury spectral lines in both the visible and ultraviolet region are generated, thereby exciting the phosphor coating on the inside of the tube to luminance.
Cathodes for fluorescent lamps typically comprise a coiled current wire and a basket wire loosely wound around the current wire. Both the current and basket wires are made of a suitable refractory material, particularly tungsten. The current wire, typically the thicker of the two wires, carries the current that passes through the cathode during operation. The basket wire is provided only to facilitate holding the emitter material in place on the cathode. Current flowing through the current wire causes the current wire to heat up, which in turn heats the emitter material to induce the emission of electrons.
Such cathodes traditionally have been manufactured by winding the wires around steel or iron mandrels and mechanically cutting the resulting wire assembly into segments of the desired length. The mandrels are removed by chemically dissolving them in an acid bath, leaving the coiled current and basket wires. Although the basket wire is wound around the current wire, the two wires are usually further connected to prevent separation while being handled in the lamp assembly process. The cathode is then covered with the emitter material, which is typically applied in slurry form.
Known ways of attaching the basket wire to the current wire include mechanical crimping, in which the basket wire is crimped or deformed against the current wire. This crimp does not metallurgically bond the two wires together, and as a result, the crimp will often let go as the finished part is handled, allowing the wires to separate. Crimped cathodes are also highly susceptible to tangling with other cathodes when stored together in a container. That is, without extreme care in the manufacturing process excessive burring can occur on the ends of the finished cathodes. These burrs tend to become entangled with the coil turns of other cathodes. This tangling makes it difficult to remove individual cathodes from the container during fluorescent lamp assembly operations.
A solution to this tangling problem is to attach the basket wire to the current wire by using a laser or plasma process to melt the full ends of the cathode, including the center steel mandrel. This forms a globular amalgam or “ball” of tungsten-iron alloy that encapsulates the basket wire and the current wire at each end of the cathode. This tungsten-iron alloy does not dissolve in the subsequent dissolving process and thus remains to secure the cathode ends. The resultant “balled-end” cathode is very resistant to tangling. However, this process requires machinery to accurately position the wire assembly in front of a laser or plasma source. While energy is applied to melt the cathode end, the wire assembly is essentially stopped to allow enough time for the material to melt sufficiently to form the large ball of the tungsten-iron alloy. This indexing is a significant limiting factor to machine throughput. In addition, a balled-end cathode results in a large portion of the product having excess retained alloy. Typically the ball of the tungsten-iron alloy consumes over 20% of the usable area.
Accordingly, there is a need for a tangle resistant fluorescent lamp cathode that can be fabricated more efficiently than balled-end cathodes.